libata: fix ata_host_start()

The loop on entry of ata_host_start() may not initialize host->ops to a
non NULL value. The test on the host_stop field of host->ops must then
be preceded by a check that host->ops is not NULL.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20210816014456.2191776-3-damien.lemoal@wdc.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/ata/libata-core.c

@@ -5573,7 +5573,7 @@ int ata_host_start(struct ata_host *host)
 			have_stop = 1;
 	}
 
-	if (host->ops->host_stop)
+	if (host->ops && host->ops->host_stop)
 		have_stop = 1;
 
 	if (have_stop) {